1. Field of the Invention
The present invention relates to a uniform illumination optical device, and more particularly, to a uniform illumination optical device with a concave lens.
2. Description of the Prior Art
Please refer to FIG. 1, which is a diagram of a prior art uniform illumination optical device 10. The uniform illumination optical device 10, applied in a projection device, is used to provide a uniform illumination of light L produced from a light source 11. The uniform illumination optical device 10 comprises a beam splitting device 12 for converting the light L from the light source 11 into a plurality of parallel beams I, and a beam condenser 14 for condensing the parallel beams I to form a more uniform condensed light L* than the light from the light source 11.
As shown in FIG. 1, the beam splitting device 12 comprises a plurality of first dispersing lenses 16, and a plurality of second dispersing lenses 18 that correspond to the first dispersing lenses 16. The first dispersing lenses 16 are flat-convex lenses used for converting the transmitted light L from the light source 11 into the plurality of beams I. Similarly, the second dispersing lenses 18 are flat-concave lenses located outside a focus of the first dispersing lenses 16, and transmit the received beams I to the beam condenser 14.
The beam condenser 14 comprises a first condensing lens 20 and a second condensing lens 22. The first condensing lens 20 is a flat-concave lens located behind the second dispersing lenses 18 of the beam splitting device 12 for receiving the beams I transmitted from the beam splitting device 12. The second condensing lens 22 is a convex lens located behind the first condensing lens 20 for receiving the beams I transmitted from the first condensing lens 20. As shown in FIG. 1, the beams I gradually converge to form the condensed light L* after passing the first and second condensing lenses 20 and 22.
Generally speaking, the light L transmitted from the light source 11 is not uniform. Therefore, the beams I generated from the light L are also not uniform. Nevertheless, the beams I are condensed to form the condensed light L*. Overall, the condensed light L* is more uniform than the light L transmitted from the light source 11.
When designing a small-sized projection device, to improve the overall performance of the device, a designer strives to achieve a short optic path, a small beam size W, and light that is parallel. A shorter optic path leads to a smaller projection device. If the beam size W is small, the actual size of the lenses, dichroic mirrors, and polarization beam splitters are decreased, which reduces the overall size of the device and lowers costs as well. In addition, dichroic mirrors, reflective liquid crystal display panels, and transmissive liquid crystal display panels all generally have a fixed range of angles for incident light. If each incident beam is parallel, the beams will have a smaller incident angle within the limiting range of allowed incident angles.
Please refer to FIG. 2, which is a diagram of an optic path for transmitted light from the uniform illumination device 10 incident upon a dichroic mirror 24. Two beams Ixe2x80x2, Ixe2x80x3xe2x80x3 generated from the light L are taken as an example. If the two beams Ixe2x80x2, Ixe2x80x3xe2x80x3 are not parallel, their corresponding incident angles xcex8xe2x80x2, xcex8xe2x80x3xe2x80x3 will be different for the dichroic mirror 24. Because the dichroic mirror 24 has a limitation on the incident angle, incident angles xcex8xe2x80x2, xcex8xe2x80x3xe2x80x3 exceeding this limitation influence the final display quality. Typically, a more expensive device, such as a coating dichroic mirror, is used to solve this problem.
Designing a short optic path, a smaller beam size, and parallel light beams according to the prior art uniform illumination device 10 is very difficult. As shown in FIG. 1, the second dispersing lens 18 is usually located behind the focus of the first dispersing lens 16, and is used to disperse the beams transmitted from the first dispersing lens 16. The optic path between the second dispersing lens 18 and the first dispersing lens 16, therefore, must be longer than the length of the focus of the first dispersing lens 16. Taking the second condensing lens 22 as an example, the curvature of the second condensing lens 22 must be large for a smaller beam size W. However, a larger curvature makes transmitted beams converge towards the optic center so that the transmitted beams are no longer parallel. The design of the prior art uniform illumination device 10 is thus incapable of simultaneously satisfying the demands for a short optic path, a smaller beam size, and parallel beams.
It is therefore a primary objective of the present invention to provide a uniform illumination device with a concave lens to solve the above-mentioned problem.
Briefly, the claimed invention provides a uniform illumination device comprising a beam splitting device for converting the light from the light source into a plurality of parallel beams, and a beam condenser for condensing the parallel beams to form a more uniform condensed light than the light from the light source. The beam condenser further comprises a first condensing lens made of a convex lens and a second condensing lens made of a concave lens. When the parallel beams transmitted from the beam splitting device pass through the first condensing lens of the beam condenser, the parallel beams converge to the central axis. On the other hand, when the parallel beams pass through the second condensing lens, the parallel beams converge with axes that are parallel with the central axis. In the end, the parallel beams transmitted from the beam splitting device through the first condensing lens and the second condensing lens mix to form the more uniform condensed light.
It is an advantage of the present invention that, with the concave lens, the uniform illumination device can satisfy the demands of having a short optic path, a smaller beam size, and the desired parallel beams.
These and other objectives of the present invention will no doubt become obvious to those of ordinary skill in the art after reading the following detailed description of the preferred embodiment, which is illustrated in the various figures and drawings.